The present invention relates to injection molding and particularly to the precise control or metering of thermoplastic material to one or more mold cavities.
The use of control units such as shooting pots to introduce thermoplastic resin into mold cavities in an injection molding system is well known. In such arrangements a primary resin source feeds the reservoir of a shooting pot and the shooting pot in turn is operated to feed a measured or metered quantity of thermoplastic material into the mold cavity.
It is also known to supply thermoplastic material to a multicavity mold where the cavities of the mold are fed by a hot runner system and where the hot runner system includes a plurality of shooting pots, with at least one each individual to each molding cavity. That is, a source of thermoplastic material leads to the hot runner system which in turn feeds the respective mold cavities wherein a shooting pot or metering means is provided for each mold cavity for controlling precisely the introduction of a shot of resin to the mold cavity. U.S. Pat. Nos. 4,717,324 and 4,775,308 teach the coinjection of a plurality of thermoplastic materials to mold an article having a layered wall structure using thermoplastic materials having different optimum processing temperatures including the maintenance of the optimum temperatures in flow paths individual to each material from its source to a mold cavity. These patents show shooting pots used in conjunction with the formation of the layered wall structure and multicavity molding. However, these disclosures involve the use of shooting pots to individually meter the amount of the first feeding of a first resin and the first feeding of a second resin into each cavity of a multicavity mold. In all cases, however, the final feeding and packing of the cavity is achieved by pressurizing the extruder or injection unit used to supply the final feeding resin to all the cavities in the mold. Thus, the hot runner supply channel system must be carefully balanced between the extruder or injection unit and each cavity gate or cavity orifice of the multicavity mold. In case of a large number of mold cavities, such as 32 or 48, this becomes quite impractical to achieve.
Accordingly, it is a principal object of the present invention to provide an injection molding method and apparatus for feeding a supply of thermoplastic material to a mold cavity from a shooting pot which can effectively eliminate the need for packing by the extruder or injection unit.
It is a further object of the present invention to provide a method and apparatus as aforesaid which is suitable for use with an injection molding apparatus including a large number of cavities.
Further objects and advantages of the present invention will appear hereinbelow.